It's unknown what caused the fatal crash, and the Federal Aviation Administration and the National Transportation Safety Board are conducting an investigation into the incident.

"There were clouds around 4,000 to 5,000 feet ... but we do not know if weather was a factor," in the crash, said Corey Chaskelson, a forecaster with the National Weather Service, to the Associated Press.

"They have taken care of so many kids in their short lives," Dr Jay Pershad, medical director of Le Bonheur's air rescue program, said of the crew who were killed to CNN. "I can't tell you how incredibly difficult it is for the entire team."

The crash is the second fatal incident in three years for the Hospital Wing rescue company. A March 2010 crash killed the pilot and two nurses after an encounter with a storm.

Founded in 1985, the Hospital Wing Unit now operates seven helicopters and flies 400 times a year, notes WPTV.com, serving patients within a 150 mile radius of Memphis.

"Lots of people count on us to go out and rescue children wherever they are," said Le Bonheur Children's Hospital president and CEO Mary Armour to WMCTV. "Ped-Flite is like the cavalry, going out into the community and surrounding areas. Our families often tell me that once the Ped-Flite team shows up, they know they're going to be taken care of."